APOLLO'S LUNAR EXPLORATION PLANS

Scientific Interest in the Moon

In the two and a half centuries since Galileo first turned his crude
telescope on the moon, astronomers have mapped its surface features,
measured the height of its mountains and the depth of its craters, and
calculated its size, mass, and orbital parameters with increasing
accuracy over the years. Even so, the best optical telescopes, under the
best observing conditions, could not show any detail smaller than about
300 meters across (1,000 feet, about the size of the capitol building in
Washington); so information concerning the nature and texture of the
surface was limited.

To the telescopic observer, the moon's surface shows two distinct types
of regions: 1. the maria (Latin = "seas"; singular
mare) - dark, apparently smooth, and roughly circular areas
extending over hundreds of kilometers; and 2. the highlands, mostly
mountainous regions much lighter in color. Among the most striking
features are the craters - tens of thousands of circular depressions
ranging in diameter from 180 miles (290 kilometers) down to the limit of
telescopic resolution. Most large craters have high walls and a
depressed floor, and many have a central peak or ridge. Some are centers
from which streaks of light-colored material (rays) extend for
considerable distances. Besides the craters, the lunar surface shows
domes, ridges, and rilles - long, narrow channels resembling dry
watercourses - that run for several kilometers.1 Measurements of reflected light indicate that
the lunar surface is covered with a layer of finely pulverized material.
Airless and arid, the lunar surface is subjected to temperature changes
of more than 200 degrees C (360 degrees F) during the course of a lunar
day.2

For decades these features fascinated astronomers and cosmologists, who
have generated volumes of speculation as to the moon's origin and
history. Three hypotheses attracted adherents: one, that the moon was
spun off from a molten proto-earth; a second, that the moon was formed
in a separate event and later captured by the earth; and the third, that
the earth and moon formed at about the same time in the same region of
space (this hypothesis was considered somewhat less likely than the
first two). Those who believed the moon was formed by accretion of
smaller bodies generally supposed that during its evolution the moon,
like the earth, went through a molten stage in which its components were
chemically fractionated, with iron and nickel being separated from the
rocky minerals as the moon cooled. A smaller body of opinion held that
the moon never grew large enough to be completely melted by the heat
generated as its component particles coalesced, and would not be
chemically differentiated.3

All of these assumptions led to difficulties when examined in light of
accepted celestial mechanics. If the moon separated from the earth at
some early stage in its formation, its orbital plane should lie in the
plane of the earth's equator, but it does not. If, on the other hand,
the moon was formed elsewhere in the solar system, its capture by the
earth would be highly improbable and its present orbit difficult to
account for. Finally, if the earth and moon were formed out of the sane
primordial matter by any mechanism, it is hard to explain the fact that
the earth is nearly half again as dense as the moon.4

None of the hypotheses could be proved or disproved on the basis of the
evidence available from visual observation alone. Both led to logical
conclusions concerning the chemical composition and internal structure
of the moon that could not be tested. Scientists who held that the moon
was once molten regarded the mafia as massive lava flows from volcanoes
or fissures in the lunar crust, similar to known examples on earth, and
pointed to the domes and certain craters that are much like well known
terrestrial volcanic structures. Advocates of the "cold moon"
theory considered the maria to be the solidified remains of large bodies
of molten rock created by collision with meteorites or asteroids, or
perhaps by localized heating due to some other cause.5 The origin of lunar craters was a matter for
debate. Some had undoubtedly been produced by impacts of cosmic debris,
but strong arguments could be made that others were volcanic in origin.
It was generally accepted that the moon's surface, unaffected by wind
and water, preserved a record of cosmic events which the earth must also
have undergone; on earth, however, the effects have been obliterated by
erosion.6

Crucial to the confirmation of either hypothesis, or to the creation of
an alternative, was information that could be obtained only by direct
examination of the moon. Analysis of samples of the lunar surface would
show whether the moon was chemically similar to the earth and whether
the lunar material had ever been extensively melted. Measurement of the
flow of heat from the moon's interior to the surface would show whether
it was still cooling from a molten state. Other important investigations
included the seismic properties of the moon, which could reveal its
interior structure. Some of this information could be provided by
instruments, perhaps including remotely controlled samplers capable of
returning lunar material to earth. But some tasks, such as examining the
moon's surface and selecting samples on the basis of that examination,
could better be done by humans. With the advent of the space age, lunar
scientists could look forward to sending instruments to the lunar
surface; only after the decision to land people on the moon could they
hope to send a trained explorer.

1. See Harold C. Urey, "The
Contending Moons," Astronautics & Aeronautics,
vol. 7, no. 1 (Jan. 1969), pp. 37-41, reproduced in "Lunar Science
Prior to Apollo 11," Robert Jastrow, Vivien Gornitz, Paul W. Gast,
and Robert A. Phinney, eds. (NASA Goddard Space Flight Center Institute
for Space Studies, 1969), pp. I-1 to I- 6. This collection, a summary of
discussions at a conference on problems in lunar science held in New
York on June 5, 1969, contains papers and excerpts from books by the
leading researchers in lunar science. Although it includes some results
from Ranger and Surveyor that are not
pertinent to the present discussion, this volume is a good single source
of pre-Apollo information on the moon.